The wide use of poly(vinyl chloride) resins (hereinafter "PVC" for brevity), because of their economic availability, is limited to those applications where thermal stability is of secondary importance. In other applications where a thermally stable resin is essential, chlorinated poly(vinyl chloride) resins (hereinafter "CPVC" for brevity), are employed. The chlorination of PVC has been studied in great detail over the past twenty years or so, and numerous chlorination processes have been developed. Most preferred is a process carried out by suspending PVC in water, which PVC is swollen with a lower halogenated hydrocarbon swelling agent, and irradiating swollen PVC with ultraviolet light while bubbling chlorine gas into the water. This process is disclosed in U.S. Pat. No. 2,996,489 to Dannis, M. L. and Ramp, F. L. Several subsequent inventions related to this basic process have been disclosed in the textbooks "Polyvinylchloride und Vinylchloride--Mischpolymerizate," pp 120-125, Springer, Berlin (1951); "Vinyl and Related Polymers," by C. A. Schildknecht (1952); and in U.S. Pat. Nos. 2,426,808; 2,590,651; 3,100,762; 3,334,077; 3,334,078; inter alia. The disadvantage of these liquid-phase processes in which the reaction occurs in a liquid medium, is that (a) chlorine dissolves in water with difficulty, and even at elevated temperature and pressure, chlorinated product forms relatively slowly; and, (b) it is only with difficulty and expense that essentially all the swelling agent used in these processes can be removed from the CPVC product.
Other processes use reaction in a liquid medium without a swelling agent, such as that disclosed in German Pat. No. 2,322,884 published Nov. 22, 1973; U.S. Pat. Nos. 3,506,637 and 3,534,013; inter alia.
Still other less preferred chlorination processes comprise dissolving or suspending the resin in a chlorinated hydrocarbon solvent and promoting the reaction with heat, light, or a catalyst. Yet other processes utilize a fluidized bed of PVC which is contacted with chlorine gas, optionally diluted with an inert gas, and optionally also containing a lower chlorinated hydrocarbon, again catalyzed by ultraviolet radiation. Such processes have been disclosed in U.S. Pat. Nos. 3,532,612; 3,663,392; 3,813,370; Japanese Pat. No. 49-45310; British patent specification Nos. 1,089,323; 1,242,158; 1,318,078; and, German Pat. Nos. 1,110,873; 1,259,573; inter alia. These fluidized bed chlorination processes occur in a gaseous reaction medium, but with difficulty, because of the slow gaseous diffusion of chlorine into solid PVC macrogranules. The term "macrogranules" is used herein to define a cluster or aggregate of randomly closely packed primary particles of polymer. A handful of macrogranules has the feel of fine sand, and are also referred to as "grains". A macrogranule of PVC or CPVC will typically have an average diameter in excess of 20 microns, with a preponderance of particles in excess of 50 microns in diameter. A preferred size distribution of each macrogranule is in the range from about 50 to about 500 microns, and conventionally ranges from about 100 to about 200 microns. Each macrogranule is made up of a multiplicity of primary particles each in the size range from about 0.05 micron to about 5 microns, and more typically in the range from about 0.5 micron (5000 A) to about 2 microns (20,000 A). The bulk of the primary particles are usually submicronic in size, though conditions of polymerization will determine the actual size distribution of both primary particles, and also, macrogranules. Macrogranules can be characterized by their porosity, that is, internal pore volume, and surface area.
The morphology of PVC and CPVC macrogranules, specifically the porosity and surface area, are important properties which determine the physical properties of the polymer after it is molded. Since CPVC is generally derived by the chlorination of PVC, it has been found that the properties of product CPVC may be tailored to a large extent by precisely controlling the conditions under which precursor PVC is polymerized. Such a process is disclosed in U.S. Pat. Nos. 3,506,637 and 3,534,103. With care, the internal morphology of PVC macrogranules may be particularly tailored to permit relatively fast chlorination in a fluidized bed process catalyzed by actinic radiation. Even so, it is necessary for economy, to practice the process in two stages, as disclosed in U.S. Pat. No. 4,039,732 to Stamicarbon B.V.
I am unaware of any process for the relatively dry chlorination of PVC macrogranules in which only sufficient liquid chlorine ("Cl.sub.2 ") is used as will "wet" the macrogranules without any visual appearance of having been "wetted". The terms "wet" and "wetted" are used herein to refer solely to the presence of liquid Cl.sub.2 on macrogranules of polymer, and not to the presence of water. When the requisite amount of liquid Cl.sub.2 within a narrowly specified range is absorbed by the solid PVC which is then irradiated with actinic (ultraviolet) radiation, there results a reaction in the solid PVC medium which chemically binds a predetermined amount of chlorine in the product CPVC. However, I am aware that it is known to chlorinate solid polyethylene ("PE") by reacting between 5 to 100 parts of liquid Cl.sub.2 per part of PE, in a reaction medium of liquid Cl.sub.2, until the resulting chlorinated PE (hereinafter "CPE" for brevity) dissolves in the liquid Cl.sub.2, and then to recover CPE by evaporating the Cl.sub.2. This process is described in greater detail in Canadian Pat. No. 471,037 to John L. Ludlow which teaches a process for the chlorination of ethylene polymers. In this process, PE is suspended in at least 5 parts liquid Cl.sub.2 (hence referred to as "the high liquid Cl.sub.2 process"), and irradiated with a suitable light source. The chlorination of PE proceeds from the surface inwardly, the chlorinated polymer dissolving from the polymer substantially immediately upon its formation, thereby exposing unchlorinated polymer. In this way the high liquid Cl.sub.2 process makes it possible to chlorinate PE polymers at a rapid rate. However, many polymers of monoolefinically unsaturated monomers are not chlorinated in liquid Cl.sub.2, or only slightly chlorinated. For example, poly(vinyl fluoride), and poly(vinylidene chloride-vinyl chloride, 88:12) are not chlorinated; and, as Ludlow taught, unless PE is suspended in at least 5 parts by weight liquid Cl.sub.2, there is very little chlorination.
Because of the essential physical difference between liquid Cl.sub.2 and gaseous Cl.sub.2 (or "vapor Cl.sub.2 "), it would not be expected, in the chlorination of crystalline and non-crystalline polymers derived from alpha-beta-monoolefinically unsaturated monomers, to obtain essentially the same freedom of access to the interior portions of the polymers when chlorinating with liquid Cl.sub.2, as when chlorinating with vapor Cl.sub.2. Expectations relative to freedom of access are further complicated and clouded depending upon (a) the physical condition of the chlorinated exterior portion of the polymer and its ability in such condition either to impede or facilitate travel of liquid Cl.sub.2 (or vapor Cl.sub.2 in a "dry" process, that is, with non-wetted polymer), and (b) the probability of the chlorinated exterior dissolving into the liquid Cl.sub.2 so as to continuously expose unchlorinated polymer. Yet, quite surprisingly, it is found that the rates of chlorination of a mass of PVC grains wetted with liquid chlorine only to the extent that it remains free-flowing with a visual indication of being dry (hence referred to as "wetted but free-flowing"), compared to dry PVC chlorinated with chlorine vapor, are close to being equal under essentially the same conditions of chlorination. This indicates that a vapor phase chlorination of dry free-flowing PVC, and chlorination of wet but free-flowing PVC are essentially analogous in reaction mechanism, and in the progress of each chlorination.
Particularly since Ludlow, supra, teaches the chlorination of ethylene polymers which are relatively crystalline, in a process in which chlorination proceeds in a liquid chlorine medium, it would be expected tht chlorination of polymers such as PVC which are relatively non-crystalline, would proceed more easily in a liquid chlorine medium than a crystalline polymer. It does. Quite unexpectedly however, chlorination of PVC also progresses satisfactorily in a solid reaction medium, this medium being solid polymer. The result is that relatively dry chlorination of a wetted but free-flowing mass of PVC macrogranules may be effected directly, that is, the relatively dry mass of PVC macrogranules is directly converted to a dry mass of CPVC macrogranules. This direct conversion of PVC to CPVC in a solid medium bypasses the problematical recovery of CPVC from a solution of CPVC in liquid Cl.sub.2.
Though vapor-phase chlorination of PVC proceeds in the gaseous phase, the quality of CPVC produced is comparable to that produced by low liquid chlorination. This similarity of product quality is thought to stem from a realization of the expectation that the rate of chlorination of relatively non-crystalline polymers, initiated by actinic light, are not diffusion limited because Cl.sub.2 molecules are free to gain access to all the interior portions of the polymer. But there was no reason to expect, prior to my discovery, that this freedom of Cl.sub.2 molecules would extend to wetted but free-flowing PVC and account for such efficient chlorination. Accordingly, in the prior art, vapor-phase chlorination processes such as are described in U.S. Pat. Nos. 3,532,612; 3,663,392; 3,813,370 and 4,039,732 are preferred.
It was not expected that a wetted but free-flowing relatively non-crystalline massive polymer would be chlorinated so as to remain free-flowing, and resist being converted to a liquid or pasty mass, or to a mass of self-adherent grains because of dissolution in liquid Cl.sub.2. Nor was it expected that desired chlorination could be effected without adversely affecting the physical characteristics so essential to the processability of CPVC in commercial manufacturing operations.